1. Field of the Invention
The present invention relates to circular-polarization antennas, and more particularly, to a compact low-profile circular-polarization antenna such as a GPS antenna used for car-mounted and portable navigation systems. The present invention further relates to a radio apparatus using the circular-polarization antenna.
2. Related Art of the Invention
FIG. 17 shows a conventional circular-polarization antenna. The circular-polarization antenna is of a microstrip patch type.
In FIG. 17, the circular-polarization antenna 100 includes a mounting substrate 101, a patch antenna 102 mounted on the second major surface of the mounting substrate 101, a shield case 103 for covering an amplification circuit (not shown) mounted on the first major surface of the mounting substrate 101, and a cable 104 connected to the amplification circuit at one end. The patch antenna 102 is made from a dielectric substrate such as ceramic and resin. A ground electrode is formed on the whole first major surface of the patch antenna 102 and a substantially square emission electrode 102a is formed on the second major surface. About between the center of the emission electrode 102a and one corner, a through hole 102b passing through the dielectric substrate and the mounting substrate 101 and being connected to the amplification circuit mounted on the first major surface of the mounting substrate 101 is formed.
FIGS. 18(a) and 18(b) show another conventional circular-polarization antenna. It is also of a microstrip patch type. The same symbols as those used in FIG. 17 are assigned to the patch antenna. FIG. 18(a) is a perspective view of the circular-polarization antenna when viewed from the second major surface of a mounting substrate and FIG. 18(b) is a perspective view of the circular-polarization antenna when viewed from the first major surface of the mounting substrate.
In FIG. 18, the circular-polarization antenna 110 includes a mounting substrate 111, a patch antenna 102 mounted on the second major surface of the mounting substrate 111, a shield case 112 for covering an amplification circuit mounted on the second major surface of the mounting substrate 111, a connection electrode 113 for connecting the patch antenna 102 to the amplification circuit through the first major surface of the mounting substrate 111, a shield case 114 for covering the connection electrode 113 at the first major surface of the mounting substrate 111, and a cable 115 connected to the amplification circuit at one end.
Since the patch antenna 102 has a microstrip structure, a ground electrode is formed substantially on the whole mounting surface thereof. To connect the patch antenna 102 to the amplification circuit, it is necessary to route through the first major surface of the mounting substrate 111. The shield case 114 is provided in order to protect the connection electrode 113 and to suppress unwanted emission.
In the conventional circular-polarization antenna 100 shown in FIG. 17, since the ground electrode is formed on the whole mounting surface of the dielectric substrate constituting the patch antenna 102, it is necessary to provide the amplification circuit and the shield case 103 at the first major surface of the mounting substrate 101. Therefore, the circular-polarization antenna has a high profile as a whole. In addition, since components are required to be mounted on both surfaces of the mounting substrate 101, mounting cost is high.
On the other hand, in the conventional circular-polarization antenna 110 shown in FIG. 18, the amplification circuit and the shield case 112 are mounted on the same plane of the mounting substrate 111 as the patch antenna 102. Also in this case, to form the connection electrode 113 for connecting the amplification circuit to the patch antenna 102 on the first major surface of the mounting substrate 111, it is required to provide the shield case 114 for covering the connection electrode 113 on the first major surface of the mounting substrate 111. Therefore, it is difficult to reduce the height of the circular-polarization antenna as a whole and mounting cost is unlikely to be reduced.
In the conventional case shown in FIG. 18, a multilayer substrate may be used for the mounting substrate and the connection electrode could be formed on an inner layer of the mounting substrate. In this case, however, the mounting substrate costs increase even further.